Improve field set builders more

[sffc]

Things to do:

• Add .with_time_precision() to date field sets, so the user can write YMD::long().with_time_precision(), getting a YMDT out the other side
• Rename .zone_z() and friends to .with_zone_z()
• Make these changes on DateFieldSet as well

[robertbastian]

Rename .zone_z() and friends to .with_zone_z()

As mentioned in the other issue, I'd prefer if we said "time zone" instead of "zone". If you're not in the time zone mindset, it might not be clear what "zone" means.

[sffc]

Do you prefer:

1. .with_zone_specific
2. .with_specific_zone
3. .with_time_zone_specific
4. .with_specific_time_zone
5. .with_specific_non_location_time_zone
6. .with_z_time_zone
7. .with_time_zone_z

Right now I believe the main branch is on 1.

People using these APIs are already in a datetime mindset and will understand what "zone" means. There will be people who are confused by "zone", but this is mitigated with docs.

To demonstrate the utility of "zone" as a self-contained term, please note the following enum and its variants.

https://unicode-org.github.io/icu4x/rustdoc/icu/datetime/fieldsets/enums/enum.CompositeFieldSet.html